In portable radio devices, mobile terminals in particular, the antenna is preferably placed inside the covers of the device for convenience. The internal antenna of a small device is usually of planar-type, because satisfactory electric characteristics are then most easily achieved for the antenna. The planar antenna comprises a radiating plane and a ground plane parallel therewith. As mobile terminals are becoming smaller thickness-wise, too, the distance between the radiating plane and the ground plane of a planar antenna should be as short as possible. However, a drawback of the reducing of said distance is that the bandwidth(s) of the antenna are becoming smaller. Then, as a mobile terminal is designed to function according to different systems having frequency ranges relatively close to each other, it becomes more difficult or impossible without special arrangements to cover said frequency ranges used by more than one radio system. Such a system pair is for instance GSM 1800 (Global System for Mobile telecommunications) and GSM 1900. Correspondingly, securing the function that conforms to specifications in both transmitting and receiving bands of a single system can become more difficult.
The above-described drawbacks are avoided, if a resonance frequency or resonance frequencies of the antenna can be changed electrically so that the operation band of the antenna round a resonance frequency always covers the frequency range, which the function presumes at a given time.
From publication JP 8242118 is known a solution for adjusting antenna's resonance frequency, such that at each side of the radiating plane there are openings extending from the edge of the plane towards the center area thereof. To each opening is connected an electronic switch which, when conducting, shorts the opening in question at a certain point. Changing the state of a switch changes electrical dimensions of the radiating plane and, thereby, the resonance frequency of the antenna. Each switch is controlled with a control signal of its own, so the antenna can be adjusted step by step. A drawback of this solution is that the effect of a single switch is minimal, and therefore many switches are needed. The number of switch components and mounting them causes remarkable extra cost.
From publications EP 0 687 030 and U.S. Pat. No. 5,585,810 is known a solution, in which between the radiating plane and the ground plane there is a capacitance diode and another capacitive element. Antenna's resonance frequency is changed by changing the capacitance of the diode by means of a control voltage via a control circuit. A drawback of this solution is that it complicates the basic structure of the antenna, in which case the manufacturing costs of the antenna are relatively high. This is emphasized in multi-band antennas, since separate arrangement is needed for each operation band.
From publication U.S. Pat. No. 6,255,994 is known a solution according to FIG. 1. There can be seen a rectangular radiating plane 2 and a ground plane 3. These planes are supported at a certain distance from each other by a dielectric block 14. At the one end of the antenna there are feed/receive conductor 4, first short conductor 5 and second short conductor 6, which conductors are joined galvanically to the radiating plane. The feed/receive conductor is isolated from the ground plane by a hole 3a, first short conductor by a hole 3b and second short conductor by a hole 3c. The first short conductor 5 can be connected to the ground plane through the first switch 7. This is a two-way switch, a terminal 7a of which can be connected to a terminal 7b or to terminal 7c. In the former case the first short conductor is connected to the ground plane through an inductive element 8 and in the latter case directly. Instead of an inductive element a capacitive element can be used or both of these can be used besides the direct connection. The second short conductor 6 can be connected to the ground plane through the second switch 9. This is a closing switch, a terminal 9a of which can be connected to a terminal 9b. In this case the second short conductor is connected directly to the ground plane. The state of the switch 7 is determined by the first control signal SD1 coming from a controller 13, and the state of the switch 9 is determined by the second control signal SD2 coming from the controller 13. The resonance frequency of the antenna structure is changed by controlling switches 7 and 9. In the case of two-state switches there are four alternative short-circuit arrangements and at the same time resonance frequencies. Three of these are used: The lowest frequency is obtained when the first short conductor is connected through the inductive element and the second short conductor is not at all connected. The higher frequency is obtained when the first short conductor is connected directly to the ground plane and the second short conductor is not at all connected. The highest frequency is obtained when the first short conductor is connected through the inductive element and the second short conductor is connected directly to the ground plane. By dimensioning the radiating plane and the distances between the conductors joined to it, the spaces between the operation bands corresponding to three resonance frequencies can be determined.
A drawback of this solution is that when a multi-band antenna is needed, it is in practice difficult or impossible to match above-mentioned operation bands to the frequency ranges used by the systems at issue. Moreover the structure comprises, compared with an usual PIFA (planar inverted F-antenna), an additive short conductor with it's arrangements, resulting to extra size and manufacturing cost of the antenna.